1. Field of the Invention
The present invention relates to a diagnostic process for use in evaluation of sensors signals, especially knock sensor output signals of a knock sensor of an internal combustion engine, and, more particularly, to a diagnostic process for evaluation of sensor signals with an evaluation device that forms a reference level from the output signals, which is compared with an upper and/or lower limiting value for error analysis, which establishes the existence of an error when the actual reference level exceeds the preset upper limiting value or falls below the present lower limiting value and which performs the diagnostic process only when a minimum detection threshold has been exceeded.
2. Description of Related Art
In sensors, for example in knock sensors, which are used for knock detection and thus for controlling knock in an internal combustion engine, it is known that a reliable diagnostic process must be performed, which guarantees that erroneous operations of the sensor are detected and faulty detection, for example erroneous detection of knock is reliably prevented.
If knock sensors are, for example, associated with the respective cylinders of an internal combustion engine, they produce output signals, which, after appropriate signal preparation and a special evaluation process, detect whether or not knock is occurring in the concerned cylinders, also whether or not knocking composition is occurring or not. Knock is a matter of undesirable combustion events, which should be avoided.
The reference PCT DE 94/01041 describes a process of knock detection including a diagnostic process. A rotation-speed-dependent standard reference level is formed by the control unit of the internal combustion engine in this known process, as is generally common in methods for knock detection, from the output signal of a knock sensor or the signals of knock sensors, especially structure-borne sound sensors. Rotation-speed-dependent reference level thresholds are formed according to this reference level. A respective upper and a lower reference level threshold is detected. Both reference level thresholds are input to the electronic memory of the control unit of the internal combustion engine as characteristic curves. The formation of the reference level occurs so that it corresponds to the sound level of the internal combustion engine during knock-free operation. The reference level or the standard reference level derived from it is also obtained with the help of an integration of the outer signals of the knock sensors at those conditions, at which not knock is detected, for example within a predetermined measurement window.
During actual knock detection in the known processes each actual standard reference level is compared with both speed-dependent reference level thresholds. Erroneous operation is detected when there is a predetermined deviation from the actual normalized reference level. When the reference level falls below the lower reference level threshold, failure of the knock sensor is established, since in this case no signal can reach the evaluation device. When the reference level exceeds the upper reference level threshold, erroneous operation of the knock sensor is established.
In order to maintain a reliable distance from the high basic noise level, which is caused by noise and irradiation in the knock sensor channel, the diagnostic process is performed above a certain minimum rotation speed in the known process. This threshold is selected so that the spacing of the standard reference level from the basic noise is so large that a reliable testing process can be performed.
It is an object of the present invention to provide an improved process for evaluation of sensors signals, especially knock sensor output signals of a knock sensor of an internal combustion engine, which can be performed reliably at comparatively lower engine rotation speeds.
This object and others, which will be made more apparent hereinafter, are attained in a diagnostic process for evaluation of sensor signals with an evaluation device that forms a reference level from the output signals, which is compared with an upper and/or lower limiting value or threshold for error analysis, which establishes the existence of an error when the actual reference level exceeds the preset upper limiting value or threshold or falls below the present lower limiting value or threshold and which performs the diagnostic process only when a minimum detection threshold has been exceeded.
According to the invention the minimum detection threshold can take two different limiting or threshold values.
The process according to the invention has the advantage that the diagnostic process can even be performed at clearly smaller engine rotation speeds than the current process according to the state of the art. Because of that feature faulty knock sensors or erroneous operation can be detected earlier so that motor damage can be reliably avoided.
Additional features and advantages are described and claimed in the appended dependent claims.